The present invention generally relates to a planar color lamp for illuminating a flat panel display and more particularly, relates to a planar color lamp that utilizes field emission light source of nanotube emitters that are arranged in serpentine shape for use in the illumination of LCD or other flat panel displays and a method for fabricating such color lamp.
In the construction of liquid crystal display (LCD) panels, a method of illumination must be utilized since the liquid crystal itself does not illuminate. The illumination is also important when the available lighting for viewing a LCD is insufficient. In order to make large LCD panels, and specifically colored LCD panels, a high efficiency light source must be used for illumination in order to achieve the requirements of small panel thickness, lightweight and low power consumption. The capability of achieving high brightness at a low power consumption is essential for obtaining a long battery life between recharging in portable applications for LCD""s. In recent years, the improvements made in the other parts of a LCD display, i.e., the color filter arrays, the thin film transistors, and other performance enhancement layers reduce the overall transmittance of a liquid crystal display panel. As a result, any improvement that can be made in the brightness/power ratio must be obtained from the improvement in the backlighting efficiency of a panel.
In the conventional backlighting technology for flat panel displays, cathode fluorescent lamps are used to illuminate the flat panel display. The cathode fluorescent lamps provide the benefits of high luminous efficiency, long service life, lightweight and rugged structure. The lamps are normally installed in pairs along the sides of a display panel, e.g., a display panel in a notebook computer, with a light tube arrangement for creating uniform lighting across a diffuser screen. More recently, improvements in backlighting have been provided which include a flat fluorescent backlight and a wedge-shaped light tube which distributes the light from a single bulb evenly over the entire display surface. The wedge-shaped construction allows a single lamp to illuminate the entire liquid crystal display panel. A plastic molded light tube which contains prismatic specular reflectors helps to spread the light uniformly across a front plane of the device.
Flat fluorescent lamps have also been recently developed to directly illuminate a display panel. A typical construction of a flat fluorescent lamp device measures only 3 mm thick. Panel sizes ranging from diagonal lengths between 25 mm and 350 mm have been made by using the conventional cold cathode technology. The lamp housing can be constructed by using a formed plate and a flat plate laminated together. For instance, a typical lamp can be constructed of a serpentine channel of four intervals equipped with an electrode at each end. A typical design of the flat fluorescent lamp includes a phosphor coating on both a top and a bottom plate, while a reflective coating is placed only on the bottom plate. A high voltage of between 1 kV and 3 kV (depending on the panel size and cathode type) is normally required to operate a flat fluorescent lamp.
For a color liquid crystal display device, color filters in three basic colors of red, green and blue must be utilized. The manufacturing process for color filters involves a number of steps such as chemical vapor deposition, spin coating of insulators and metals, and the planarization and orientation film coatings. Color filters can be formed on glass substrates by complicated processing steps which include glass finishing and preparation of both the front and the back of a substrate, the polishing and lapping process, the washing and cleaning of the substrate, the coating, curing and other steps which must be performed on the substrate.
The formation of color filters requires a repetitive process to be carried out for forming the three primary color elements. Inbetween the color elements, a black border or a black matrix is needed for providing the necessary contrast. To prepare the color filters, either an organic dye or a pigment can be used as long as it is suitable as a light absorbing color filter material. For instance, a gelatin can be deposited and dyed in successive photolithographic operations by using proximity printing equipment and standard photoresist materials. A pigment dispersion method can also be used which eliminates the gelatin layer and is capable of higher temperature stability. Other methods for forming color filters include electrodeposition and printing.
FIG. 1A shows a conventional color filter device 110 consisting of three primary color filters, i.e., red filter 112, green filter 114 and blue filter 116. A white light source 120 is used for backlighting the single pixel 110. In this conventional color filter/backlighting arrangement, a large area is occupied by a single pixel and as a result, the resolution achieved on a liquid crystal display panel is relatively poor.
In another conventional color filter/backlighting device as shown in FIG. 1B, in the same area that was occupied by a single pixel where a white light backlighting is used, three pixels are arranged wherein each pixel can be one of the three primary colors by utilizing three different light sources 124, 126 and 128 for each pixel. Significant improvement in resolution is therefore possible due to the greatly reduced sizes of the pixels. The color filters used in this arrangement, 130, 132 and 134 are essentially transparent for accepting a color from the color sources 124, 126 and 128. This arrangement is known as a sequential color display. In the sequential color display arrangement, a cathode-ray tube is normally employed as a light source that emits light at a plurality of wavelengths. Since there is an inherent light loss created by the polarization of the emitted light and the duty cycle of the liquid crystal cell, the maximum efficiency for the transmitted white light is reduced to as low as 25%. The display brightness in a field sequential color display is therefore a major concern.
In recent years, flat panel display devices have been developed and widely used in electronic applications such as personal computers. One of the popularly used flat panel display device is an active matrix liquid crystal display which provides improved resolution. However, the liquid crystal display device has many inherent limitations that render it unsuitable for a number of applications. For instance, liquid crystal displays have numerous fabrication limitations including a slow deposition process for coating a glass panel with amorphous silicon, high manufacturing complexity and low yield for the fabrication process. Moreover, the liquid crystal display devices require a fluorescent backlight which draws high power while most of the light generated is wasted. A liquid crystal display image is also difficult to see under bright light conditions or at wide viewing angles which further limit its use in many applications.
Other flat panel display devices have been developed in recent years to replace the liquid crystal display panels. One of such devices is a field emission display device that overcomes some of the limitations of LCD and provides significant advantages over the traditional LCD devices. For instance, the field emission display devices have higher contrast ratio, larger viewing angle, higher maximum brightness, lower power consumption and a wider operating temperature range when compared to a conventional thin film transistor (TFT) liquid crystal display panel.
A most drastic difference between a FED and a LCD is that, unlike the LCD, FED produces its own light source utilizing. colored phosphors. The FEDs do not require complicated, power-consuming backlights and filters and as a result, almost all the light generated by a FED is visible to the user. Furthermore, the FEDs do not require large arrays of thin film transistors, and thus, a major source of high cost and yield problems for active matrix LCDs is eliminated.
In a FED, electrons are emitted from a cathode and impinge on phosphors coated on the back of a transparent cover plate to produce an image. Such a cathodoluminescent process is known as one of the most efficient methods for generating light. Contrary to a conventional CRT device, each pixel or emission unit in a FED has its own electron source, i.e., typically an array of emitting microtips. A voltage difference existed between a cathode and a gate electrode which extracts electrons from the cathode and accelerates them toward the phosphor coating. The emission current, and thus the display brightness, is strongly dependent on the work function of the emitting material. To achieve the necessary efficiency of a FED, the cleanliness and uniformity of the emitter source material are very important.
In order for the electron to travel in a FED, the FEDs are evacuated to a low pressure such as 10xe2x88x927 torr in order to provide a long mean free path for the emitted electrons and to prevent contamination and deterioration of the microtips. The resolution of the display can be improved by using a focus grid to collimate electrons drawn from the microtips.
In the early development for field emission cathodes, a metal microtip emitter of molybdenum was utilized. In such a device, a silicon wafer is first oxidized to produce a thick silicon oxide layer and then a metallic gate layer is deposited on top of the oxide. The metallic gate layer is then patterned to form gate openings, while subsequent etching of the silicon oxide underneath the openings undercuts the gate and creates a well. A sacrificial material layer such as nickel is deposited to prevent deposition of nickel into the emitter well. Molybdenum is then deposited at normal incidence such that a cone with a sharp point grows inside the cavity until the opening closes thereabove. An emitter cone is left when the sacrificial layer of nickel is removed.
In an alternate design, silicon microtip emitters are produced by first conducting a thermal oxidation on silicon and then followed by patterning the oxide and selectively etching to form silicon tips. Further oxidation or etching protects the silicon and sharpens the point to provide a sacrificial layer. In another alternate design, the microtips are built onto a substrate of a desirable material such as glass, as an ideal substrate for large area flat panel displays. The microtips can be formed of conductive materials such as metals or doped semi-conducting materials. In this alternate design for a FED device, an interlayer that has controlled conductivity deposited between the cathode and the microtips is highly desirable. A proper resistivity of the interlayer enables the device to operate in a stable condition. In fabricating such FED devices, it is desirable to deposit an amorphous silicon film which has electrical conductivity in an intermediate range between that of intrinsic amorphous silicon and n+doped amorphous silicon. The conductivity of the n+doped amorphous silicon can be controlled by adjusting the amount of phosphorous atoms contained in the film.
Generally, in the fabrication of a FED device, the device is contained in a cavity of very low pressure such that the emission of electrons is not impeded. For instance, a low pressure of 10xe2x88x927 torr is normally required. In order to prevent the collapse of two relatively large glass panels which form the FED device, spacers must be used to support and provide proper spacing between the two panels. For instance, in conventional FED devices, glass spheres or glass crosses have been used for maintaining such spacings in FED devices. Elongated spacers have also been used for such purpose.
Referring initially to FIG. 2A wherein an enlarged, cross-sectional view of a conventional field emission display device 10 is shown. The FED device 10 is formed by depositing a resistive layer 12 of typically an amorphous silicon base film on a glass substrate 14. An insulating layer 16 of a dielectric material and a metallic gate layer 18 are then deposited and formed together to provide metallic microtips 20 and a cathode structure 22 is covered by the resistive layer 12 and thus, a resistive but somewhat conductive amorphous silicon layer 12 underlies a highly insulating layer 16 which is formed of a dielectric material such as SiO2. It is important to be able to control the resistivity of the amorphous silicon layer 12 such that it is not overly resistive but yet, it will act as a limiting resistor to prevent excessive current flow if one of the microtips 20 shorts to the metal layer 18.
A completed FED structure 30 including anode 28 mounted on top of the structure 30 is shown in FIG. 2B. It is to be noted, for simplicity reasons, the cathode layer 22 and the resistive layer 12 are shown as a single layer 22 for the cathode. The microtips 20 are formed to emit electrons 26 from the tips of the microtips 20. The gate electrodes 18 are provided with a positive charge, while the anode 28 is provided with a higher positive charge. The anode 28 is formed by a glass plate 36 which is coated with phosphorous particles 32. An intermittent conductive layer of indium-tin-oxide (ITO) layer 34 may also be utilized to further improve the brightness of the phosphorous layer when bombarded by electrons 26. This is shown in a partial, enlarged cross-sectional view of FIG. 2C. The total thickness of the FED device is only about 2 mm, with vacuum pulled inbetween the lower glass plate 14 and the upper glass plate 36 sealed by sidewall panels 38 (shown in FIG. 2B).
The conventional FED devices formed by microtips shown in FIGS. 2Axcx9c2C produce a flat panel display device of improved quality when compared to liquid crystal display devices. However, a major disadvantage of the microtip FED device is the complicated processing steps that must be used to fabricate the device. For instance, the formation of the various layers in the device, specifically the formation of the microtips, requires a thin film deposition technique utilizing a photolithographic method. As a result, numerous photomasking steps must be performed in order to define and fabricate the various structural features in the FED. The CVD deposition processes and the photolithographic processes involved greatly increase the manufacturing cost of a FED device.
In a copending application, Attorney""s Docket No. 64,600-050, assigned to the common assignee of the present invention, a field emission display device and a method for fabricating such device of a triode structure by using nanotube emitters as electron emission sources were disclosed. In the triode structure FED device, the device is constructed by a first electrically insulating plate, a cathode formed on the first electrically insulating plate by a material that includes metal, a layer formed on the cathode of a high electrical resistivity material, a layer of nanotube emitter formed on the resistivity layer of a material of carbon, diamond or diamond-like carbon wherein the cathode, the resistivity layer and the nanotube emitter layer form an emitter stack insulated by an insulating rib section from adjacent emitter stacks, a dielectric material layer perpendicularly overlying a multiplicity of the emitter stacks, a gate electrode on top of the dielectric material layer, and an anode formed on a second electrically insulating plate overlying the gate electrode. The FED device proposed can be fabricated advantageously by a thick film printing technique at substantially lower fabrication cost and higher fabrication efficiency than the FEDs utilizing microtips. However, three separate electrodes are still required for the device, i.e., a cathode, a gate electrode and an anode.
It is therefore an object of the present invention to provide a planar field emission color lamp that utilizes nanotube emitters which does not have the drawbacks or shortcomings of the conventional color lamps.
It is another object of the present invention to provide a planar field emission color lamp that utilizes nanotube emitters which does not require the fabrication of complicated microtip electron emitters.
It is a further object of the present invention to provide a planar field emission color lamp that utilizes nanotube emitters wherein the nanotube emitters are formed of nanometer dimensioned hollow fibers that are electrically conductive.
It is another further object of the present invention to provide a planar field emission color lamp that utilizes nanotube emitters as electron sources wherein the nanotube emitters can be advantageously formed by a thick film printing technique.
It is still another object of the present invention to provide a planar field emission color lamp that utilizes nanotube emitters wherein the nanotubes are provided in a material of carbon, diamond-like carbon or diamond.
It is yet another object of the present invention to provide a planar field emission color lamp that utilizes nanotube emitters for emitting electrons which are formed in a serpentine-shape on a base insulating plate of the lamp.
It is still another further object of the present invention to provide a planar field emission color lamp that utilizes nanotube emitters as the electron source and fluorescent coating strips formed in serpentine-shape on a cover plate for activation by the electrons emitted from the nanotube emitters.
It is yet another further object of the present invention to provide a method for fabricating a planar field emission color lamp with nanotube emitters by a thick film printing technique such that serpentine-shaped nanotube emitters and fluorescent coating strips can be formed on a base plate and on a cover plate, respectively of the color lamp.
In accordance with the present invention, a planar field emission type color lamp that is equipped with nanotube emitters and a method for fabricating such color lamp are provided.
In a preferred embodiment, a planar field emission color lamp that utilizes nanotube emitters can be provided which includes a lamp body that has an electrically insulating cover plate, an insulating base plate, two side walls and two end walls forming a sealed cavity therein, at least three spaced-apart, serpentine-shaped emitter stacks formed on the electrically insulating base plate, each of the at least three serpentine-shaped emitter stacks being positioned substantially parallel to the two end walls and is formed by a layer of a first electrically conductive material and a layer of nanotube emitter on top, a layer of a second electrically conductive material on a surface of the electrically insulating cover plate that faces the cavity, at least three spaced-apart, serpentine-shaped fluorescent coating strips on the layer of the second electrically conductive material corresponding in a mirror image relationship to the at least three emitter stacks when the cover plate is positioned over the base plate forming the lamp body, each of the at least three fluorescent coating strips is adapted for emitting a red, green or blue light upon activation by electrons emitted from the at least three emitter stacks, and a plurality of electrically insulating spacers inbetween the cover plate and the base plate for maintaining a preset spacing thereinbetween.
In the planar field emission type color lamp that has nanotube emitters, the layer of nanotube emitters may be formed of a mixture of nanometer dimensioned hollow tubes and a binder material. The electrically insulating cover plate may further include a black matrix layer inbetween the at least three spaced-apart, serpentine-shaped fluorescent coating strips. The black matrix layer may be formed of an electrically conductive material for dispersing charges carried by the electrons when bombarded on the black matrix layer. The cover plate and the base plate may be formed of a ceramic material that is substantially transparent. The layer of a first electrically conductive material may be a cathode for the planar field emission color lamp. The layer of a first electrically conductive material may be a silver paste, while the layer of the second electrically conductive material is an anode for the planar field emission color lamp and may be formed of indium-tin-oxide (ITO). The layer of nanotube emitter may be formed of a mixture of nanometer dimensioned hollow tubes of carbon, diamond or diamond-like carbon and a polymeric-based binder. Each of the at least three fluorescent coating strips emits a light of red, green or blue that is different than the light emitted by its immediate adjacent strips when activated by electrons from the at least three emitter stacks.
The present invention is further directed to a method for fabricating a planar field emission color lamp that utilizes nanotube emitters which can be carried out by the operating steps of providing an electrically insulating base plate, forming at least three spaced-apart serpentine-shaped emitter stacks on the electrically insulating base plate by a thick film printing method substantially parallel to a transverse direction of the base plate, each of the at least three emitter stacks includes a layer of a first electrically conductive material and a layer of nanotube emitter on top, providing an electrically insulating cover plate, forming a layer of a second electrically conductive material on a surface of the electrically insulating cover plate facing the electrically insulating base plate when the cover plate and the base plate are assembled together, forming at least three spaced-apart, serpentine-shaped fluorescent coating strips on the layer of the second electrically conductive material corresponding to the positions of the at least three emitter stacks for emitting a red, green or blue light when activated by electrons emitted from the emitter stacks, and joining the electrically insulating base plate and cover plate together by side panels forming a vacuum-tight cavity therein.
The method for fabricating a planar field emission color lamp with nanotube emitters may further include the step of providing the cover plate and the base plate in substantially transparent glass. The method may further include the step of printing the layer of the first electrically conductive material in a silver paste. The method may further include the step of screen printing the layer of nanotube emitters from a mixture of a binder and nanometer dimensioned hollow fibers selected from carbon fibers, diamond fibers, and diamond-like carbon fibers. The method may further include the step of providing a negative charge to the first electrically conductive material underlying the plurality of emitter stacks and providing a positive charge to the layer of second electrically conductive material. The layer of the second electrically conductive material may be formed of indium-tin-oxide. The method may further include the step of coating a black matrix layer on the electrically insulating cover plate inbetween the at least three strips of fluorescent coating. The at least three fluorescent coating strips may be formed by a thick film printing technique. The at least three fluorescent coating strips may be formed such that each strip emits a red, green or blue light that is different than its immediate adjacent strips when activated by electrons from the plurality of emitter stacks. The method may further include the step of forming the at least three fluorescent coating strips by a material that includes phosphor. The method may further include the step of forming six spaced-apart, serpentine-shaped fluorescent coating strips for emitting color lights in the order of red, green, blue, red, green and blue.